Microstructural Evolution and Ductile-to-Brittle Transition in a Low-Carbon MnCrMoNiCu Heavy Plate Steel
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A structural steel should not be used at temperatures lower than the ductile-to-brittle transition temperature (DBTT), at which the steel loses most of its toughness and fractures in the brittle cleavage mode.[1–3] In the course of cleavage fracture, transgranular cracking takes place along the typical cleavage of the {100}a plane. The fracture stress rf is an intrinsic mechanical property pertaining to the cleavage fracture and is insensitive to temperature. The increase in rf can lower the DBTT, as demonstrated by the Yoffee diagram.[1–3] The engineering significance in studying the variations in rf with microstructural factors and the scientific fascination in clarifying micromechanism of the cleavage fracture in high-strength steels have stimulated substantial research works[2,4–8] The microstructural
DONGSHENG LIU, MI LUO, and BINGGUI CHENG are with the Institute of Research of Iron and Steel, Jiangsu Shagang Group Co., Ltd, Jinfeng Town, Zhangjiagang, Jiangsu Province 215625, P.R. China. Contact e-mail: [email protected] RUI CAO and JIANHONG CHEN are with the State Key Laboratory of Gansu Advanced Processing and Recycling of Non-ferrous Metallic Metals and Key Laboratory of Non-ferrous Metal Alloys of the Ministry of Education, Lanzhou University of Technology, Lanzhou 730050, P.R. China. Manuscript submitted November 2, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
units controlling the rf were different for different steels: ferrite grains in polygonal ferrite + pearlite steels, bainitic, or martensitic packets in quenched and tempered steels, or the prior austenite grains in the as-quenched steels.[2,6,8] A Griffith form can be employed to link the rf and microstructural factors, i.e., grain size or packet size, and the effective energy of the fracture surface, cp. Hanamura et al.[2] found that cp could be quantified as 7.7, 34.6, and 150.9 J/m2 for ferrite+pearlite, tempered martensite, or the as-quenched martensite, respectively, in a JIS-SM490 steel. cp of 233.3 J/m2 was reported for a high-strength bainitic steel.[6] The local cleavage fracture stress rf decreases as the packets size increases, showing a linear dependence on the inverse square root of the size.[7,8] The decrease of rf made cleavage fracture easier, increased the DBTT, and worsened the toughness in the lower shelf region. The effect of microstructural variants, including prior austenite grain size, bainitic/martensitic packet size, distribution of martensite–austenite (MA) constituents, and the density of high-angle grain boundaries (HAGBs), on the DBTT of the steels has been revealed.[9–16] It has been believed that the presence of MA constituents is deleterious, as it increases DBTT.[9,16] The prior austenite grain boundaries,[2,6] or the boundaries that separate different colonies of the martensitic or bainitic microstructure, could be HAGBs. These boundaries inhibit the propagation of cleavage cracks.[10,11] Furthermore, it has been suggested that Bain unit
boundaries[12–14] could prevent cleavage crack propagations in martensitic s
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